1. Technical Field
The present invention relates to a reflective liquid crystal projector.
2. Related Art
Presently, reflective liquid crystal projectors are known as a kind of liquid crystal projectors. The reflective liquid crystal projector includes, for example, a light source, a polarization beam splitter (referred to as “PBS” hereinafter), a reflective liquid crystal panel, and a projection optical system. A light beam having been emitted from the light source is transmitted through the PBS and strikes the reflective liquid crystal panel. Typically, the PBS is positioned at an angle of 45° relative to a normal direction of the reflective liquid crystal panel. The light beam having struck the reflective liquid crystal panel is modulated and reflected by the reflective liquid crystal panel. The light beam having been reflected by the reflective liquid crystal panel strikes again the PBS to be split into a polarized light component that indicates an image and a different polarized light component that indicates a reversed image. The polarized light component that indicates the image is projected onto a screen or the like with the projection optical system. Thus, the image is displayed.
With the reflective liquid crystal projectors, a high contrast ratio and high luminance of a projected image can be achieved compared with those achieved with transmissive liquid crystal projectors. The reflective liquid crystal projectors have been widely used for home theaters, digital cinemas, digital signage, and so forth, where the above advantages can be made use of. However, with regard to further improvement of the contrast ratio, the fact that a polarization beam split function of the PBS depends on incidence angles has become a major obstacle.
The light beam having exited from the reflective liquid crystal panel is not typically a collimated light beam but a diverging light beam having divergence of, for example, about an angle of 20°. When the PBS splits the polarized light component of incident light having an incident angle in a range of 45°±10° without sufficient precision, the image is displayed in a state where part of the polarized light component that indicates the reversed image is not split from the polarized light component that indicates the image. This makes it difficult to improve the contrast ratio.
A related art PBS is constituted by two rectangular prisms, each of which has an inclined surface coated with a dielectric multilayer film. The two prisms are attached to each other at the inclined surfaces. Recently, wire grid PBSs have been in use. The wire grid PBSs feature a polarization beam split function that is less dependent upon incidence angles than that of the related art PBSs, thereby significantly increasing the contrast ratio of the reflective projectors.
However, since even the wire grid PBSs are dependent upon incidence angles, a super high contrast ratio (for example, 1:50000) cannot be easily achieved. JP-A-2006-113282, JP-A-2007-212997, and JP-A-2005-516249 disclose techniques that utilize an aperture stop to limit divergence of a light beam exiting from a reflective liquid crystal panel for improving the contrast ratio.
JP-A-2006-113282 discloses a technique that limits angles at which a light beam strikes a PBS. According to JP-A-2006-113282, angle characteristics at a PBS are improved and a projected state of an image in which the contrast is emphasized can be achieved by narrowing a light beam, and a projected state of an image in which a large amount of light is projected and brightness is emphasized can be achieved by widening the light beam.
According to JP-A-2007-212997, attention is focused on visual angle characteristics of liquid crystals, and an aperture stop having a cross-shaped aperture is used. This aperture stop is narrowed from four corners of the aperture in diagonal directions. With this aperture stop, according to the document, light in a visual angle direction where the contrast ratios are low can be blocked.
According to JP-A-2005-516249, aperture stops of an elliptically shaped aperture and a cross shaped aperture are used, with which the contrast ratio can be improved with a minimum reduction in brightness.
The techniques disclosed in the above documents may improve the contrast ratio. However, improvement is still required as described below in order to further increase the contrast ratio with a minimum reduction in brightness.
According to the technique disclosed in JP-A-2006-113282, if an incidence angle at the PBS is limited by the aperture stop, the displayed image becomes dark as is the case with the ordinary technique. This makes it difficult to achieve a projected image featuring both high contrast and high luminance.
According to the technique disclosed in JP-A-2007-212997, angular light components that should be shielded in terms of the visual angle characteristics of the liquid crystals are not necessarily the same as angular light components that should be shielded in terms of the incidence-angle dependency of the PBS. Therefore, there is the possibility that the high contrast may not be achieved because of insufficient separation of light that indicates a reversed image. In addition, a ratio of shielding of light indicating the image may be increased and, accordingly, the brightness may be reduced.
According to the technique disclosed in JPA-2005-516249, the polarization beam split function of the wire grid PBS is not affected by incidence angles (refer to paragraph 0039 in the document). Therefore, it is difficult to suppress the reduction in contrast ratio due to the incidence-angle dependency of the wire grid PBS. As a result, there is the possibility that the high contrast cannot be achieved. JP-A-2005-516249 also describes that light is preferentially blocked at both ends of a beam of light to a variety of degrees (refer to paragraph 0026 in the document), although this description is not consistent with the description in paragraph 0039. However, what kind of aperture stop can compensate for the polarization beam split function of the wire grid PBS is not described. Therefore, there still is the possibility that the high contrast cannot be achieved and the brightness is reduced due to the reason that is the same as that of JP-A-2007-212997.